I. Field of the invention
The present invention relates to performing signal handoff in communication systems, such as wireless data or telephone systems, using satellites. More particularly, the invention relates to a method and apparatus for handing off user terminal communication links between different satellite beams associated with a single communications satellite, or sectors in a single cell.
II. Description of the Related Art
A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users, such as code division multiple access (CDMA) spread spectrum techniques. CDMA techniques in multiple access communication systems are disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters", and U.S. patent application Ser. No. 08/368,570, filed under the title "Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy," which are both assigned to the assignee of the present invention, and incorporated herein by reference. These patents disclose communication systems in which communication signals are transferred through satellite repeaters and gateways, or terrestrial base stations (also referred to as cell-sites or cells).
In a typical spread-spectrum communication system, one or more preselected pseudorandom noise (PN) code sequences are used to modulate or "spread" user information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communication signals. PN spreading is a method of spread-spectrum transmission that is well known in the art, and produces a communication signal with a bandwidth much greater than that of the data signal. In the base station- or gateway-to-user communication link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different base stations or over different beams, as well as between multipath signals. These codes are typically shared by all communication signals within a given cell or beam, that are on a common frequency (sub-beam).
In a typical CDMA spread-spectrum communication system, channelizing codes are used to discriminate between different users within a cell or between user signals transmitted within a satellite sub-beam on a forward link (i.e., the signal path from the base station or gateway to the user transceiver). That is, each user transceiver has its own orthogonal channel provided on the forward link by using a unique `channelizing` orthogonal code. Walsh functions are generally used to implement the channelizing codes.
Wide band CDMA techniques permit problems such as multipath fading to be more readily overcome and provide a relatively high signal gain. However, some form of signal diversity is also generally provided to further reduce the deleterious effects of fading and additional problems associated with acquiring and demodulating signals in the presence of relative user, or repeater, movement, which along with large distances causes substantial dynamic changes in path lengths.
Generally, three types of diversity are used in spread spectrum communication systems, including time, frequency, and space diversity. Time diversity is obtainable using repetition and time interleaving of signal components, and a form of frequency diversity is inherently provided by spreading the signal energy over a wide bandwidth.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links with a user through two or more base stations or antennas, for terrestrial-based repeater systems; or two or more satellites or satellite beams, for space-based repeater systems. That is, for terrestrial systems signals can be transferred through multiple base stations, or more likely, through multiple antennas servicing various cell sectors. For satellite communication systems, path diversity is typically obtained by transferring signals over multiple paths using either multiple satellites (repeaters) or multiple transponder beams on a single satellite. However as discussed below, the latter approach is not generally useful.
Examples of using path diversity in multiple access communication systems are illustrated in U.S. Pat. No. 5,101,501 entitled "Soft Handoff In A CDMA Cellular Telephone System," issued Mar. 31, 1992, and U.S. Pat. No. 5,109,390 entitled "Diversity Receiver In A CDMA Cellular Telephone System," issued Apr. 28, 1992, both assigned to the assignee of the present invention, and incorporated herein by reference.
Typical spread spectrum communication systems also contemplate the use of a "pilot" carrier signal as a coherent phase reference for gateway- or satellite-to-user and base station-to-user links. That is, a pilot signal, which typically contains no data modulation, is transmitted by a base station or gateway throughout a given region of coverage. A single pilot is typically transmitted by each gateway or base station for each frequency used, typically referred to as a CDMA channel, or sub-beam. This pilot is shared by all user terminals receiving signals from that source. This provides signals that can be readily distinguished from each other, also distinguishing between beams and cells while providing simplified acquisition and tracking.
Pilot signals are used by subscriber units to obtain initial system synchronization, and provide robust time, frequency, and phase tracking of transmitted signals. Phase information obtained from tracking a pilot signal carrier is used as a carrier phase reference for coherent demodulation of communication system or user information signals.
Pilot signals are also generally used to gauge relative signal or beam strength for received communication signals. In many systems, pilot signals are also generally transmitted at a higher power level than typical traffic or other data signals to provide a greater signal-to-noise ratio and interference margin. This higher power level also enables an initial acquisition search for a pilot signal to be accomplished at high speed while providing for very accurate tracking of the pilot carrier phase using relatively wide bandwidth, and lower cost, phase tracking circuits.
As satellites transit in their respective orbits, the beams they project onto the Earth move relative to users, periodically changing which satellites can provide service for particular users. This occurs for example as satellites come into or disappear from "view". The same effect also occurs between beams in a single satellite, with service for particular users changing as the beams move across the earth's surface. In addition, mobile users sometimes move relative to beams or satellite paths, also causing beam coverage or service areas to change. In these situations, communication links for signals must be handed off between beams. A similar process occurs for terrestrial cellular systems where users move relative to base stations and sectors or sector boundaries within cells.
A basic technique developed to prevent loss of signal and improved transfer of information is the so-called "soft" handoff scheme which is described in U.S. Pat. No. 5,101,501, referred to above. In this technique, a new link or signal path is established through a new satellite, or satellite beam, before the existing or old link is disconnected or discarded. The information (energy) available for a given communication signal from each path can be combined to provide improved signal reception, as well as prevent disconnected communication links. This can be done for either the forward link communications from gateway-to-user terminal, or the reverse link communications from user terminal-to-gateway. For the reverse link, the diversity combining process is accomplished at the gateway or within a centralized control or switching center.
Unfortunately, when using soft handoff techniques in satellite communication systems several problems arise. While diversity can be used to improve signal characteristics for communication links involving multiple satellites, it is not useful for communicating to a user through multiple beams on a single satellite. Beams from a single satellite have virtually the same path at the same frequency on a forward link, with nearly the same transit time, and have the same fading or interference characteristics. Diversity combining two such forward link signals provides little benefit, while unnecessarily consuming power and adding to the background noise level or interference.
Users can also traverse between adjacent beams quickly and move back and forth along their respective boundaries. If a user is moving along the Earth's surface perpendicular to the direction of sweep for a satellite spot containing a series of beams, the user might traverse between two adjacent beams repeatedly. In this situation, a user can switch between adjacent beams on a frequent basis, especially where the beams are near the edge of coverage for a satellite spot. In addition, other factors such as low satellite elevation and local terrain or signal blockage continuously impact signal quality. In this situation, the communication system may be continuously switching between beams in a soft handoff mode to maintain a best communication link.
A similar process may occur for mobile users moving around in sectored cells in terrestrial communication systems. That is, where the cells are subdivided into two or more smaller service areas which are covered at differing frequencies or using different code spaces. Here, mobile users may travel along or repeatedly cross sector boundaries within a cell, depending on such factors as cell and sector size and local physical environment. The resulting switching activity may be increased by the use of techniques meant to otherwise increase cell capacity. For example, a cell may employ a series of relatively small sectors or sectors having adjustable sizes to increase capacity or accommodate certain traffic patterns relative to the cell service area. However, smaller sectors and more sector boundaries increase the likelihood of more frequent handoffs between sectors. Changing sector sizes may also shift a user terminal back and forth between adjacent sectors with a minimum amount of physical movement.
This switching activity tends to consume excessive system resources in several ways. First, the time spent establishing links and selecting channels, with corresponding signal time, frequency, and phase tracking, error detection, and so forth, consumes signal processing resources which could be applied to other tasks such as signal demodulation, diversity combining, and decoding. Second, for a substantial period of time, multiple orthogonal channels in each beam are in use by a single user. That is, orthogonal codes in adjacent beams, or sectors, are allocated to a single user. Since there are a relatively limited number of such orthogonal channels available in the communication system, this decreases effective system capacity. Third, additional power is consumed maintaining each active channel for a single user, double for two channels, and energy deposited into such communication channels causes interference, which is deleterious to system operation.
Therefore, what is needed is a handoff technique which allows a soft handoff between adjacent beams from a single satellite with decreased system resources when the user is traversing between such beams. The technique should also address soft handoff between adjacent sectors within a cell serviced by a base station or cell-site. The method should provide a solution that decreases unnecessary consumption of system resources while remaining compatible with other soft handoff schemes.